1. Field of the invention
The present invention relates to a digital pulse width modulation device, more particularly to a pulse width modulation device having an actual output frequency of its output pulse duty determined by setting the total length of the pulse duty, and an actual output duty cycle ratio determined by setting the width of the pulse duty.
2. Description of Related Art
In general, pulse width modulation (PWM) control system is used for controlling an electronic device by the steps of detecting an output signal transmitted to a load to obtain a sensing signal, comparing the sensing signal with a reference signal, applying different duty cycles to a pulse depending on the result of comparison to obtain a pulse width modulation signal, and transmitting the pulse width modulation signal to the load to serve as a control signal. In other words, an appropriate width modulation of the pulse signal is generated by a negative feedback of the original output signal, and the width modulated pulse signal is used for controlling and outputting an appropriate output signal to the load.
In a conventional analog pulse width modulation control system, a sensing signal Vsense and a reference signal Vref are sent to a comparator, and a compare signal Vcmp is outputted, and then the output signal of the comparator is compared with a ramp signal Vramp by another comparator, and finally a constant frequency is outputted, wherein a pulse signal VPWM capable of changing the duty cycle is used for controlling the load.
However, the frequency of the ramp signal Vramp is unstable, and thus the control system may be interfered by noises easily. Although a resistor and a capacitor can be installed onto the comparator for frequency compensation, these external components incur additional cost.